This invention relates generally to systems for driving frequency converters, and more particularly, to a system wherein electrical valves within the converter are addressed by firing signals which correspond to predetermined reference voltage and frequency values for producing at output terminals of the converter a voltage space vector which is determined from a combination of possible discrete vectors.
In a known arrangement, a constant DC input voltage is provided at DC input terminals of a converter. Such a known converter is also provided with at least one output terminal which is connected to one or more electrical valves or switches which can be addressed by firing signals such that the positive and negitive polarities of the DC input voltage are selectably applied, in response to the firing signal, at the output terminal. Thus, by providing an appropriate firing signal, the output terminal of the converter can be connected selectably to either the positive or negative polarity of the DC input voltage. If the converter switches are operated rapidly in response to the firing signals, it is possible to produce at the output terminal of the converter a selected voltage waveform, illustratively a continuous, approximate sinusoid. Such an approximate sinusoid is produced by the succession of rapidily occuring voltage pulses having respective durations which depend upon the desired output voltage. Circuitry can be provided for smoothing the pulse sequences to achieve a continuous voltage waveform.
In embodiments where the converter provides a plurality of output voltages at respective output terminals, each such output voltage can be individually smoothed. Moreover, the output voltages can be arranged to be phase-shifted with respect to one another to produce, for example, a three-phase supply. When such a three-phase supply is connected to a load, the phase voltages can be combined into a characteristic vector which corresponds to the voltage state of the load. Such a load may be a stator winding of a rotating-field machine, thereby corresponding to a three-phase symmetrical load having a neutral terminal which is not connected to a reference potential. In this known three-phase arrangement, the arrangement of converter switches in the converter can assume only eight switch combination states, thereby allowing only a finite number of possible states of the resulting space vector. Two such states correspond to situations where all of the outputs are connected to one of the positive or negative DC input terminals such that all output voltages are in the same state. The remaining six states correspond to respective three-dimensional space vectors which can be addressed by associated combinations of firing signals.
A definite voltage state is unambiguously assigned to each of the eight possible converter switch combinations to avoid inverter short-circuit states during transitions of the firing signals. As noted, six discrete space vectors are produced for each of six respective firing signal combinations, as well as two zero state combinations for the firing signals where the output terminals are shorted to each other via a selectable one of the DC input terminals. A quasi-continuous control over the output voltage can be achieved by pulsing back and forth between adjacent ones of the discrete vectors so that a desired vectorial average is produced which approximates a desired output voltage, illustratively for controlling a rotating-field machine. One known pulse control system utilizes a sine-triangle modulation method for forming control signals which provide the quasi-continuous control over the phase voltages. Such control signals are produced with respect to reference values by generating in an input stage a sinusoidal reference voltage for each output of the inverter, thereby producing a constant revolving voltage space vector. A generator provides a running time signal which may be a triangle voltage which is either free-running or synchronized with the rotating voltage space vector. Each such sine voltage is interrogated with the triangular voltage in a comparator stage, and, in the event of equality therebetween, the DC input voltage is connected with reversed polarity to the respective output terminal of the converter by changing the firing signal for the converter switches which are associated with the particular output terminal. If the frequency of the running time signal is sufficiently high, an approximately sinusoidal phase voltage can be produced by smoothing the output pulses which are combined to form the voltage space vector which is to be quasi-continuously controlled.
It is, therefore, an object of this invention to provide a simple and inexpensive controller for a pulse converter which is particularly adapted to digital operation. Digital operation permits relatively high accuracy to be achieved at low cost.
It is a further object of this invention to provide a control system for a pulsed inverter wherein the harmonic content of the output voltage is low.